Photographic emulsions and methods



MaS' 28, 1945* G. B. 'cRousE ETAL PHOTOGRAPHIC EMULSIONS AND METHODS `Filed sept. 26. 1939 Georg t ATTORNEY Los EXPOSURE Patented May 2s, 1946 y PHOTOGRAPHIC EMULSIONS AND METHODS George B. Crouse, East Setauket, Francis A. Holt, Port Jeierson, and Jacob L. Jatlow, New York, N. Y., assignors to Photo-Positive Corporation, New York, N. Y., a corporation of New York Application September 26, 1939, SerialNo. 296,536 l l1 Claims. (Cl. 95-7) Our present invention concerns light sensitive emulsion layers or coatings Yfor photographic plates, films, paper or the like, which upon proper.

exposure and rst development, give direct positive pictures of the object.

The present-day art of photographing, that is, taking pictures," depends largely upon the use of salts which tum dark'when exposed to light and then developed, and the greater the amount of light, the darker the effect. The resuit is a negative from which a positive, or

real picture may be produced by a number of well known methods. Our present invention is the result of discoveries we have made whereby light-sensitive salts'may be used to produce direct positives.

The above and other facts with respect to our invention, and the nature and objects thereof, may be better understood from the following description in connection with the accompanying drawing, in which Figs. l, 2, and 3 are curve sheets," in all of which the abscissae represent the log exposure,

i. e.. log (10) (light intensity time); the ordinates represent density of the image; and the curves represent exposures and resulting densities characteristic of certain emulsions hereinafter referred to.

In the usual emulsion layers, the-light-sensii tive. elements are minute grains or crystals of silver halides, such as silver bromide, suspended ingelatin or the like. Upon exposure within the normal limits indicated by the curve between l and 2, Fig. l', some o f these grains, to a depth and in numbers proportional to the time and intensityl of the light falling on them, are changed to form an invisible "latent image" in and among the unchanged grains.

This invisible image is potentially a negative" and, in ordinary photography, this invisible negative is subjected tof the action of a "developer" solutionl whereby the minute light-affected. grains are reduced to minuteparticles of metallic silver which appear black and,` by contrast with the lighter unchanged vparts of the layer.

- they constitute a' visible negative image oi ,the

object from which the light was projected. Then said lighter, unchanged crystals are dissolved out is to "print the negative on a second light-sensitive emulsion paper, plate or nlm, using the negative as avtransparency. The greater light projected through the more transparent parts 5 of the negative will print shadows and the less light through the denser parts will print the higher lights. y

There are also indirect reversal processes that depend upon exposing and developing the ordinary negative; then, instead of dissolving the unexposed grains as usual, the negative image itself is dissolved, leaving only said unexposed grains. These unexposed grains correspond to the high lights and low lights of the original l5 object, and the positive is produced by giving these unexposed grains a uniform all-over exposure, and then developing and iixing them, in the ordinary way. Thismethod requires a specially. thin and uniiorm emulsion layer and the processing requires veryg precise control, but it has been used as an indirect method of processing motion picture lms for amateurs.

The above indirect methods have no bearing on our invention, because they require production of a good negative, as the basis for subsequent A treatment to produce the positive picture.

Other methods, which do not require Aproduc- -tlon of a negative, `are those depending on Herschel effect. According to these methods, U0 the production of the negative image is minimized or prevented by initially fogging the emulsion. The fogging should be sumcient to pro-- duce, upon development, a density equal to the greatest density required in the nished picas ture, that is, to or near the point 2, Fig. 1, the

fogging being produced either by all-over uniform pre-exposure to white light; or by chemically fogging the emulsion during manufacture. The thus iogged plate is then used to take pictures by exposure in the usual way, but the positive,` the Herschel effect, depends on using a iilter that admits only light of long wave length,4

usually red light. 'I'he exposure required to produce the Herschel eilect is, however, so great as to preclude ordinary practical uses of these methods. y

-The solarization effects to which our invention. relates. Yresemble the Herschel" eil'ects in some ways, but diner from them in several vitally important particulars. They are similar in that a negative of the object is not necessary. so the emulsions may be prepared with a view solely to. getting the best positive-producing 4conditions As concerns the normal range oi' negative exposure, from I to 2. Figs. 1 and 2, we may sity, one after the other. With et near maximum density of a negative, as for red light Herschel eilects, but this is entirely unnecover exposure of part or all oi the positive latentv image, to whatever extent may he desirable, either for softening contrast or for standardizing the density of one positive with respect to other pic- 4 2,401,051 have our emulsions fogged or preexposed to or time of exposure required to 'produce `them is enormously greater, sometimes thousands of times greater, than the exposure required to produce the ordinary negative and the density range and contrast eiects in the final positive picture are much too far below commercial standards.

Our invention enablesy the production of emule sions capable of solarization upon relatively short exposure to ordinary white light, and,

l through intensive study and investigation, we

tures with which it may be associated,` as in pho'- togravure.

A dierence of even greater importance is that instead of being limited to red light'for our exposres, we expose to all light from the object regardless ci wave length, and our emulsions are sensitive to the blue and violet rays, like ordinary negative emulsions.

The "soiarization edect has been known as a have produced emulsions which may be solarirfed to produce positives by exposure to white light, in as short or shorter time than that required for ordinary commercial copying emulsions that produce only negatives.

This is indicated by the legends o'n Fig. 2, where the reversal exposure" curve 2-3 shows the decreasing densities that result in a positive picture, and the normal exposure curve I-2 shows the increasing densities that result in a negative.

InA this connection, it is to be noted that when y our emulsions are entirely unfogged as we preresult of grossover-exposure ola negative-producing'iilm or plate, namely, when the emulsion is developed the resulting image is a positive instead of a negative. During this exposure, the light rst produces the usual invisible negative, and its developable density increases to the maximum ci' which that 'emulsioniscapabla as indicatedby the curve between I and 2, Fig. l. When the high light portions of the negative reach that mainmum density, any further 'exposure as indicated by the line 2--2x, Fig. i, cannot produce any substantial increase above said al. i ready attained maximum.

r, continuejd exposure operates to bring the weaker-light shadow portions, up to the same mum 'dennegative-producing emulsions the entire negative is eventually brought to maximum density and if developed it would appear uniformly black` all over. But if the over-exposure is continued long enough before development, usually thousands or times longer f l to produce the ordinary negativathe ligh will have a .reversing effect on some ofthe crystals or grains constituting the now uniformly-dense, latentimage negative as indicated at 3, Fig. l; that is to say,

some of those of the already exposed crystals that are sumciently actedon by the light will be changed back again to. their original condition to suchan extent that. like uneed crystals, they will not be aected by the developer," and remaining soluble will be washed out in the fixing bath. The number of crystals thus yreversed and washed outy 'will be greater `according as the amounts of light received from different parts 'or the object are greater: so the washedbut parts will correspond to thehigh lights of the object. The unreversedl crystals are whatis left of the previously uniform latent-image after. subtracting the reversed' crystals that correspond to l the high lights, so when the emulsion is develope'd to change these latent-image remnants to the real image, and the reversed crystals corresponding to the high lights from the object have been washed out, said real image remains as shadows and the washed out parts as highlights of a directly produced positive picture.

Buch reversal eects obtainable by sola-.rization" e o! ordinary negatives, arewholly unsatisfactory for practical purposes. because the fer to have them, or partly fogged, as they may be when we use speed-increasing sensitizers, the first part of the exposure as indicated by the curvesv i-2, Fig. 2, will result in production of'a negative which will be more or less perfect according to the degree of the fog. Atthe time when the high lights in the negative reach maximum density, indicated at the peak 2, the lower lights are of the lower densities indicated along the curve at the left of said peak, correspondingto the lesser intensities of the light from the lower ylight portions cfthe object. In other words,

for the shadows where the light is less intense, f

- indicated in the region at 2, Fig. 2, before the successively lesser lights representing the shadows reachmaximum density and commence to reverse, and reversal oi the crystals corresponding to said lesser lights will commence only after correspondingly greater lengths of time. This is as it should be, because the more intense the light, the greater will be the number of maximum density crystals that must be reversed and made soluble so as to be dissolved out and leave the proper low densities necessary for a proper positive picture of the jhigh-iight parts of the object, when the layer is developed and fixed.

From` the above, it may be seen that speciiic Y,objects of our invention include the production of direct positive emulsions having speed, density range and contrast equal or superior to the standard negative-producing emulsions now commonly employed for copying, emerging, etc.

Some of the advantages oi' our invention are: (a) After proper exposure, a direct positive results on first development; (b) the cest is no greater than that of a single negative; (c) lit may be processed in the same manner and with the same simplicity as the present negative emulsion; (d) for copying, a positive is always desirable and sometimes essential; (e) emulsions having widely different speed and contrast characteristics may be prepared for-*different purposes; and withany given emulsion, contrastmay be controlled in a considerable degree by varying contrast to be -developed yon the same Aiilm or For the sake of clarity, the following definitions of` various terms employed herein are given:

E-exposure-is the product of light intensity (usually stated in terms of meter-candles) and time (usually stated in seconds).

reciprocal of the transmission of light through the silver deposit.

Transmission i`s the ratio of the transmitted light to the incident light.

Fog is theldensity produced by a given devel-` opment on those portions of the emulsion which have received no exposure to light.

vContrast of an emulsion is generally designated by the Greek symbol gamma and it is the tangent of the straight line portion of the curve. In other words, referring to Fig. 2,

v g d (density) Gaiuinab -d m- Photographically and chemically considered,`

the invention has numerous aspects of varying scope and importance. One novel feature is the preferred strongly acid condition of our solutions during formation of the light-sensitive grains in.

connection with .which some oi'lour important discoveries have been originally made. VAlkaline precipitations and mildly acid precipitations. have been used in connection with manufacture of ordinary' negative-producing emulsion, but we have discovered that with'proper acid concentration at the time of precipitation, or ilrst digestion. or both, emulsions which otherwise would be normal, negative-producing emulsions reverse and vproduce direct positives, when developed f after properly prolonged exposure; that the reversal tends to improve as acidity is increased; that with acidity below pH\4.7 good reversing be as long as several hours.

negatives may be produced; and that with acidity pH 3.5 to pH 2.5, .highly satisfactory reversingqualities may be produced. y We have also found that the acid concentration maybe further increased to pH values well below v2.5 until 'such strength of acid concentration is obtained as results in undesirable reduction of the ratio between maximum and-minimum density ofthe silver deposited on development.

The ratio of maximum to minimum developable density should be at least 4 for most purposes. In other words, it is generally desirable that -v Dinox 'I DE@ v Emulsions made in accordance with our in. vention are capable of attaining this and submarked speed of reversal and desirable contrast as above noted. n l

Furthermore, from numerous experiments we have formulated certain theories which experience shows can be used as guides governing production lof emulsions involving our present invention, which whileY recognizing and including our aciditydiscoveries, are broad enough to in- 3 clude a wide range of pH values, higher than any o! those above mentioned, and extending into the range of dennite alkalinity. Such theories,

applicable as guides will be more fully explained hereinafter.

Our methods of making direct positive' emulsions in accordance with the present invention,

presuppose knowledge and use of the ordinary' 'technique of emulsion making,'and in order to understand the present novel variations thereof, w.

it may be noted that practically all photo emulsion coatings involve utilizing certain generic procedures, which will now be briefly described:

Most of the steps are generically the saine for either silverchloride or silver bromide negativeproducing emulsions. A gelatin solution is prepared by soaking the gelatin in cold water, then vheating it and dissolving therein a suitable quantity of ammonium halide, such as chloride or bromide, and the whole kept warmin a water bath.

Precipitation: A solution of silver nitrate is then introduced, with constant stirring, the ammonium halide and silver nitrate reacting to 'precipitate a silver salt. Ammonium bromide,

for instance, vwill precipitate silver bromide, and form ammonium nitrate which remains in solution. The silver bromide is the potentially light sensitive compound, but not very sensitive.

First digestion: This is accomplished by heat treating the emulsion for 'a period which may The increase of sensitivity during this process is connected with the solubility of the grain-forming silver broat precipitation it is mide in the ammonium bromide, and to increase this solubility, some ammonium bromide additional to that required for-.the reaction is customarily added before precipitation.

Secondary gelatin: This is usually added after first digestion is completed.

Chilling, shredding and washing: This is for the purpose of freeing the emulsion from the soluble salts; such as the excess ammoniumV bromide and also the ammonium nitrate formed during precipitation. l

The chilling forms a pudding-like gel. Shredding is. by forcing the gel through a ilsh net or wire screen to form a mass of strings. Washing is usually for several hours, in a running stream of tap-water.

, After-ripening,v or "second digestion: This is eiected by melting the washed emulsion and heat treating it, usually at constant temperature, for a period which is`determined empirically. Physically considered. this is like the llrst digestion, but most of the soluble salts having been removed by the washing. chemical sensitizers" including those naturally in the gelatin or those specially added, can act to form the usual light-sensitive silver sulphide nuclei on or in the silver halide crystals.

Coating. The emulsion is then coated orV spread in a thin layer on a suitable support,

plate, nlm or paper, and coating may be facili-` stantially greater density ranges, vin addition to tated by modifying .the consistency of the emulsion, by addition of certain doctorathat is, .j photographically inertsubstances, such as alcohol, saponin, etc.4

Many variations in the above materials and steps are common in the art. For instance, slow emulsions vused for contact printing, may be vmade with silver chloride instead o!V silver bro mide; high speed emulsions generally contain a small percentage of silver iodide; halides or able' to adsorptionby the silver able for non-reversing chloride the ine and other metals may be used in place of the ammoniumsalt; and centrifuging may be employed in lieu of chilling, shredding, and washing.

The steps used in the manufacture of our direct positive emulsionsmay be generically the same as thoseA above described for the usual negative emulsions, but a forming of Qthe grains during precipitation and/or. irst digestion is under conditions favorhalide grains, or halogen or other substance capable of destroying developability oi said grains. Such adsorpis aided by having the grain-forming precipitation and rst digestion both take place in a solution containing a soluble halide and also electro-positive ions, preferably hydrogen ions, preferably sufficient to make the solutions deiinitely acid.

The halogen is ordinarily present during precipitation and first digestion in the form of a sol- I uble halide. The electro-positive ions, preferably hydrogen ions, are introduced by adding an acid to the soluble halide solution, preferably before precipitation; or by adding a soluble salt.

Starting with a formula appropriate for the production oi good negatives when made up as first above described, but with the addition of acid as above noted, an emulsion may be formed which, unless over-washed or otherwise unfavorably modified bythe after-treatment, will have 'the characteristics shown in Fig. 2. That is to the region marked normal say, if it is exposed in exposurel region, a negative image will result. but ii.' the exposure values lie in the range marked reversal exposure region, result..

Examples of our emulsions employing acids to supply electro-positive hydrogen ions to acidity the solutions to somewhere around pH 3.5 or lower are as follows:

Foam I Solution A: Y

Gelatin gms 20 Normal hydrochloric acid. I .ml-- 22.6 Ammonium chloride gms-- 2.6 Water to makem`l 150 Solution B: Silver nitrate gms 3.85 Water to make ml.-

novel feature isthat the a direct positive will chlorine derived from the hydrochloric acid). The halide concentration after precipitation and during the first digestion was .31 molar, repre-- the case of substances such, for example, as am- (l) Heat Solution IA to deg. C. and add Solution B, drop by drop, with constant stirring. Time of precipitation, approximately 10 minutes. (2) Digest two hours, at 45 deg. C, (3) Add 12.3 ml. 10% solution neutral chrome alum. (4) Chill, shred` and wash with nine baths of .ve minutes each, water acidifiedto pH 3 with hydrochloric acid. (5) Second digestion, about d hours, at 45 deg. C. (6) Doctors such as alcohol, etc.,- may be added.' at this point. ('I) Coat to approximate equivalent of ver per square meter.

In this and all or the other formulae herein specied the weight of gelatin referred to is the air-dry weight. v

The above is an example of how formulae suitemulsions, may be modied to impart strong reversing qualities. merely vby adding enough hydrochloric acidA so that precipitation and In the foregoing emulsion the pH was 2.8 during first digestion. The' halide concentration before precipitation was 47 molar (including the rst digestion occur in a solution acidiled to below pH 4.7.

monium or potassium halides, the molar concentration, of course. is equal tothe normal concentration, the relation between molar and normal concentrationsbeing well understood.

The wash water was acidiiied so that it contained halogen. and hydrogen ions suflicient to prevent reducing the halogen and hydrogen ions in the emulsion-below a certain predetermined minimum. This gave greater latitude for longer washing, without danger of impairing -the reversal qualities ofthe finished emulsion.

Such emulsions resemble ordinary chloride emulsions in that if exposed only long enough to produce a negative, the blacks and whites of the image are exceptionally' pure and have a wide density range, and the same thing is true of the positives produced by the somewhat longer exposure lrequired for reversal. Reversal speeds may be about the same as'for Well known Azo papers. In the present case, exposure for contact printing a positive was about 1 minute, with a 10i) watt lamp, about 18 inches from the paper.

. FoRMULAII Solution A1 -Gelatin gms-- 4 Normal hydrobromic acid m1 5 Ammonium bromide gms 12.48 Water to make L ml-- 150 Solution B: x L

Silver nitrate gms 10 Water to make ml-- 110 Solution C:

Ammonium bromide gms-- 5.76 Water` to make -ml 110 (5) Chill, shred and Wash with nine baths, of five minutes each, with water acidied to pH 3 by hydrobromic acid. (6) Digest at 55-60 deg. C.

4 gms. metallic silto maximum reversal sensitivity. The digestion time may be as long as ten hours. ('l) Add doctors if required. (8) Coat to approximate Vequivalent of 4 gms. metallic silver per square meter The above is an example of a more or less conventional lformula for a bromide emulsion, plus acidification as in Formula I, except that for Athe bromide', the acid is 'hydrobromid instead of hydrochloric. Wash water is also acidied, as in the chloride formula. Desirable diierences are that in precipitating Solutionv A the gelatin is greatly decreased, and is preferably only sufiicient to suspend the bromide of silver. A further important diherence is that the excess ammonium bromide in Formula II is considerably greater than commonly employed in bromide xormulae.

The pI-I of'this emulsion was 2.6 during first digestion. I'he halide concentration was .88 normal before precipitation and .36 normalduring `iirst digestion.,

The neutral chrome alum was added in the emulsions of Formulae I and II to prevent excessive swelling of the gelatin during washing. However, the neutral chrome alum may be omitted where, as in Formula III, presently to be described, the acid is neutralized after first digestion, or where, as in later formulae (IV and IX),

no acid is present. It may also be omitted where the washing is reduced (as in V and VII, later referred to) or where the emulsionY (as in subsequent Formulae VI and VIII) is subjected to centrifuging in lieu of washing. Curtailing the amount of washing or raising the pH dispenses with the need of the above-mentioned treatment with neutral chrome alum.

In practical performance, this bromide emulsion' seems to have reversing speed' and qualities differing from the above chloride reversing emulsion, somewhat as ordinary negative-producing bromide emulsions differ from negative-producing chloride emulsions.

In this Formula II, the

4 .potassium hydroxide solution.

Wash water is acidied to pH 3, for the' same reasons as in Formula I.

In formulae like I, and II, merely V(1) precipi-` tating in prese' )e of the acid, without rst digestion, washing or second digestion; or (2) precipitating without acid and then adding the acid and giving first digestion, will result in an emulsion, which without washing or second digestion, will reverse and produce a direct positive;. but 3) having the acid present during both precipitation and first digestion, gives unexpectedly great improvement in reversal qualities of the emulsion the grains of such intimately associated material,

will greatly improve the quality of the direct positive picture that can be produced, and this may be accomplished by washing in acidiled water, as described. Moreover, the acid having served its primary useful function, may be neutralized. The emulsion may then be given limited washing in unacidiiied water as notedv in connection with Formula III, presently to be described.

Second digestion after washing, is desirable,

especially in the case of bromide emulsions, as in Formula II, because of its very beneficial effect in modifying reversal contrast and raising maximum obtainable density.

FORMULA III Solution A: I ,y

i Gelatin gms ,2

Ammonium bromide gms-- 12.48 Potassium iodide gms l .08 Normal hydrobromic acid m1 5 Water tomake ml Solution B:

Silver nitrate -gms 10 Water to make 'ml 110l Solution C; V l y Ammonium bromide gms-- 5.76

Water to make ml.- .110 Il Y Solution D:

Bromine ..gms- .00 Water to make --ml r5 (1) Heat Solution A to 65 deg. C., and Solutions B and C to 70 deg. C. and add, with constant stirring, Solutions B and vC to A. (2) One minute after precipitation is started, add Solution D rapidly from a pipette. Total time of precipitation, approximatelylfive minutes. (3) Digest for two hours at 55-60 deg. C. (4) Add 38 gms. secondary gelatin, swelled in cold water.

(5) Neutralizethe acid by adding 5.0 ml. normal and wash with nine baths, 5 minutes each, water. (7) Melt and add .017 gm. thiosinamine (allyl thiocarbamide) in about 2 mls. water. (8) Digest aty 55-60 deg. C. to maximum reversal sensitivity. This may require 7 to 10 hours. (9) Add doctors as required. (10) Coat to approximate equivalent of 4 gms. Metallic silver per square meter.

Theabove emulsion for direct positives `is like that of Formula II, as concerns differences from conventional bromide formulae, but it has other differences. The added iodide is conventional, but using only half as much gelatin during precipitation and adding bromine one minute after precipitation is started have been found to fur- .ther facilitate and increase the adsorption of bromine in `or on the silver bromide crystals, thereby improving reversal qualities.

Another difference is utilization of our discovery of the possibility, and the desirability of neutralizing the acid after adding gelatin then washing in pure. water; vand limiting the washingto avoid unduly denuding the halide grains and to create favorable conditions for the thiosinamine. Our .017'A gram thiosinamine for l40 grams, dry weight of gelatin, is a ratio of 1 in 2400, as contrasted with A1 in 50,000,which is considered,about the limit that can be used in negative-producing emulsions, without producing objectionable fog. We found that even with our 1 in 2400 ratio, the fog density on development without exposure was only about 0.6, measured bythe Martens sensitometer; while reversal exposure and development of a positive, shows no fog at all.

An important practical point 'is that speeds are remarkably increased and its positive-producing reversal speeds lie in the same range as the negative-producing speeds of ordinary bromide papers. This is as fast as could be desired for copying with a camera, projection printing, or similar uses. Even for outdoor camera use, 2 seconds exposure will be sufficient for a good positive if the light is good.

In the emulsion .produced according to Formula III as above described the pH was approximately 1.2 during rst digestion. The halide concentration was .98 molar before precipitation and .37 molar during first digestion. As abov'enoted, the acid was neutralized after first digestion. The pH during second digestion was 6.1.

The following is a formula for a direct positive emulsion,'in which a l metal is used in place of' the hydrogen during precipitation and rst digestion.

Forms IV Solution A: i

' Gelatin r. gms-- 2 Ammonium bromide ,gms 12.48 Potassiumiodide ..-gms .08 Nickel chloride (NiClz) gms..I .8

Water to make-- ...ml-- -140 (6) Chill, shred the above formula, and its manufacture are closely parallel with Formula III. Consequently, it has the benefit of closely limited washing and other factors favoring adsorption of bromine as described in connection with Formula III, but the positive ions present at precipitation are nickel from the nickel chloride instead of hydro-l gen from hydrobromic acid, and the emulsion not having been rendered definitely acid by the nickel chloride, it is unnecessary to neutralize by potassium hydroxide. or equivalent.

The resulting emulsion, as-made by us, has

speed, density range, etc. not so good as Formula III, but better than Formula II.

The pH of the Formula IV emulsion was 6.1 during first digestion. The concentration of -halide before precipitation was 1.0 normal and during iirst digestion was .39 normal.

It willbe noted that the four formulae given above, as well as those set forth hereinafter, are by way of illustration and example, and that many variations maybe made within the scope of our invention. As an assistance to others in determining other specific formulae `within that scope, we will referto the theories, which we lhave found useful as guides in applying and extending the principles of our invention to include various other illustrative formulae.

These guides are based on facts, some heretofore established by others, and others of greater present importance -established experimentally by ourselves', as follows:

(1) It has been established, by other investigators:

. (a) that a photographic emulsion may be regarded as composed of three parts: first, the

SolutionB: the eifect of light, as is shown by the fact that Silver nitrate --.gms 10 the direct positive image may be developed by the Water to make. -ml-.\. ilo action of the same kinds of developing solutions Solution C: f which produce ordinary negatives; land which Ammonium bromide gms.. 5.76 5 may be used to produce negatives in our emulsions Y Water to make ml 110 when not exposed enough to produce reversal.

Solution D: (6) Unlike ordinary negative emulsions, our

' Thiosinamlne -gms-- .01'1 direct positive bromide emulsions will build lup i Water-to make (about) .ml..- 2 very little density for either a positive or a nega- For illustrative purposes and direct comparison, l0 tive, when exposed and developed immediately after washing; second digestion greatly increases the possible maximum density particularly for bromide emulsions.

(7) An indication as to how the continued action of light operates to reverse the grain, is based on the fact that the free halogens attack nely dispersed metallic silver with the formation of the halide of silver and that this action takes place more energetically in the presence of hydrogen ions.

In view of the above discoveries, we have formulated certain working hypotheses which have served us as guides for varying conditions and constituents so as to get the best results, particularly as concerns speed,l

density range, contrast, etc.

(A) When the silver halide grain is precipitated in the presence of halogen ions, or is digted in the presence of these' ions, the halogen becomes intimately associated with the grain, probably by adsorption, or sblid solution. It may be adsorbed to the surface of the grain, or to the surfaces of the tiny mosaics of which the crystal is made up; and where acid is used, it may be that hydrogen is actually substituted for silver at certain lattice points in the completed crystal. It is not necessary `to be more specific about it,

y and we shall use the term adsorption to cover sensitive silver halide grain; second, a strongly adsorbed layer of gelatin surrounding the grain;- and third, the bulk of the gelatin in which the grains are suspended: and it is now generally accepted. j

(b) that the latent negative image is a tiny sub-microscopic speck of metallic silver on the surface of, each grain, brought about by the photo-chemical reduction of the silver halide and that when lacted on by the developer, this speck serves as the vulnerable starting point for complete reduction of the entire grain.

'(2) From the evidence, resulting from our numerous testslv we believe that the `same things are true of our emulsions so far as concerns producing negatives.

(3) The reversal, that is, the direct positive result, is evidently brought about by light of the same wave lengths that' produce the ordinary negative.

(4) Just as the formation of the ordinary nega'- tive latent image is an effect of light on each individual grain, independently of the presence or Y behavior of neighboring grains, so reversal is anV eet of-lightron each individual-grain independently of the others. Y

(5) The reversal is like the negative in that it is not a chemical eect of thendevelopcr. but is any of these cases where halogen and hydrogen or other ions are intimately associated with the silver halide .and in or adjacent to each crystal,

as distinct from and additional to that which may be diffused in the gelatin.

(B) After rst digestion, when (as in Formula III) the concentration of the hydrogen in the bulk of the gelatin is reduced by neutralizing, and the concentration of bromine is reduced by washing, remnants of both hydrogen and bromine are retained by the adsorbed layer of gelatin surrounding the crystal; but the grain, including the crystal and said adsorbed layer, remains substantially insensitive to light, and. at

this stage, a layer of the emulsion will not build up density even after prolonged exposure. As fast as" the light acts to form the latent image silver speck in or on one of the grains, the bromine and hydrogen retained in the adsorbed layer of gelatin attackv the silver speck and recouvert it into silver-bromide.

(C) The above reaction which nullifles the speck forming reaction during first digestion, is modified during second digestion by two simultaneous processes. The layer of adsorbed gelatin,

and possibly the extreme outer shell of the crystal itsel!,is slowly denuded of most of its adsorbed reversing material by diffusion and consequent dilution thereof throughout the bulk of the gelatin: while at the same time the emulsion gains in normal sensitivity by formation of the usual silver sulphide specks` that constitute the sensitivity nuclei. As a result of these two processes during second digestion, the emulsion gains in both normal and reversal speed and in the ability to build up density in the properly exposed regions.

(D) As a result, the silver speck-forming and and many other facts and silver speck-reducing effects that were simultaneous during irst digestion, are rendered separable and successive by second digestion. That is to say, upon exposure of our reversing emulsions,

we conceive that the negative latent image speck is first formed in the ordinary way, and it will cause the grain to be developabie as a negative: if the exposure is not carried beyond this point.

As the light continues to act, however, the size of the latent image speck will increase. The silver bromide grain vis necessarily partially dec0m posed to increase 4the metallic silver in the speck,

thereby liberating bromine, and both brom'ine and other ions are liberated from the material ad.

sorbed incr on the grain. As the latent image speck is thus increased, a point is reached where the displaced but closely adjacent bromine becomes sufficiently concentrated to attack the silver speck and reverse it back to silver bromide. This be reduced to silver to form the dense parts of a negative; Vand will be dissolved out in the xing solution, thus leaving relatively thin areas that v constitute the higher-light parts of a direct posi'- tive picture of the object.

(E) In connection with the above, it may be noted that most theories of the formation ofthe latent image postulate the presence of a bromide acceptor, which prevents the re-combination of the silver and bromine and consequent regression of the latent image. In our direct positive emulsions, the presence of adjacent excess halogen (and hydrogen if acid is used) may be viewed as having the eiect of overloading or reducing the capacity of the acceptor so that regression of the latest negative image, -that is, so-called reversal is directly brought about by action of the light, if the exposure is sumciently great.

(F) Whether or not our views as to the mechanism ofthe reversal are correct and complete, it, seemsl abundantly clear, from our work, -that the conditions necessary for producing our reversing emulsions, run parallel with-the conditions necessary to produce strong adsorption of halogen by the sensitive silver halide grain.

(G) The presence of the hydrogen or equivalent ions, aids the adsorption of bromide or equivalent halogen by the silver halide grain, and it is possible that this is the only reason why the presence of acid is so desirable at the time the halide grains are being formed,v or reformed, but the fact that the attack of the halogens on colloidal silver is more energetic in the presence of acid, and that we have so far been unable to detect an electric charge on'the grains of-our direct positive emulsions, leads us tov believe that hydrogen, as well as halogen, is adsorbed by the grain and takes an active part in the destruction of the latent nega'- tive image during the reversal exposure.

(H) We have used the -terms hydrogen ions and halogen ions, because it appears more probable, on general theoretrical grounds, that 'these elements are present in the ionized form; but we.y

use these terms as the basis for our practical working hypotheses and not as essentials of our invention, for the very good reason thatwe have no direct proof, and it is unnecessaryto be sure whether or not the hydrogen halogen, bromine, etc which we use are present in the charred state.

. natant solution. and then pour oil.' the liquid and pick up the remaining material in:

Water ml 687 v Potassium chloride gms 1.5 Gelatin gms-- 54 Digest 4 hours at 46 C. pH during vilrst digestion 2.8 Halide concentration before precipitation y molar 1.34 After precipitation .48 pH during second digestion. 5.5 Halide concentration during second digestion .034

u 4 Alternatively. the above formula my be picked Further examples ofy direct-positive producing emulsions embodying our invention are as follows:

. Fonmli V Solution A:

Ammonium chloride gms 10.4 Water 'mi v 492 Normal hydrochloric acid mL- Gelatin ..-gms- Temperature "C 64 solution B:

Silver nitrate. gms 20 Water ml 40 Temperature C 25 Add Solution B to A in 10 minutes. Digest 1% hours at 63 C. Chill and store in refrigerator over-night (storage temperature of about 15 C. is suitable).

Shred and wash. ve baths, ilve minutes each. Digest 2 hours at 46 C. Coat. pH during nrst digestion 3.6. Halide concentration before precipitation .47 molar.' After precipitation .2 5. hese ilgures include chlorine derived from the ac I Halide concentration after washing .1l molar. The emulsion produced from this formula as above described has substantially the characteristics of that produced from Formula I.

FORMULA VI Solution #1: v

Potassium chloride gms-- 39.2 Water ml..` 450 Potassium bromide gms-- 13.1 Normal hydrochloric acid m1 87.5 Gelatin gms Temperature C 75 Solution #2:

' Silver nitrate gms. 75 Water ml 75 Temperature C 75 Add Solution #2 to Solution #l in three minutes.

First digestion, 5 hours, 75 C.

Centrifuga sufficiently to produce a clear super- This emulsion has a reversal speed approximately `four times that of the chloride emulsion of Formula1 or Formula- V, while retaining a high contrastlgamma equalling about 5). As a result it is particularly advantageous for the contact printing of letters or other documents.

The above-specified emulsion may be termed a 25% broma-chloride, since the bromide is pres-A ent inan amount sumcient to combine with about 25% of the silver nitrate.

up after centri g in the rollo, for use on tracing cloth for reproducing of plans, etc.

Ass solution #2 to solution #1 m sus minutes. First digestion ilve hours, '75 C. Y

Then add-'- Y Solution #3: i i

Water ml 336 Gelatin m 62.5 'Normal sulfuric acid ml 23 Chill and store in refrigeratory over-night.4 Shred and wash, six baths. live minutes each.

.Halide concentration before precipitation l molar 1.2 75 versal properties of the emulsion are Halide concentration during seconddiges-x tion molar-- .13

This emulsion (a 70% momo-chloride) is suitable for pictorial copying. It three times that oi the emulsion of Formula VI and a gamma value ofabout 1.

Forms VIII Solution #1: Ammonium bromide gms 30 Water ml 260 Potassium iodide gms .16 Gelatin gms 3.3 Normal hydrochloric acid rnl.- 5 Ethyl alcohol ml 5 Temperature C` 95 solution #2:

Silver nitrate gms Water m1-- 67 Temperature C 95 Add solution @c2A to solution #1 in ove min- Di'gest 10 minutes at 95 C.

centrifuge to produce ac lear supernatant liquid and pick up the solid material in- Solution #3:',

Ammonium bromide gms .13 Water mi 220 Gelatin gms 40 v Normal hydrochloric acid ml 22 Potassium-sulfocyanate gms-- .32 Thiosinamine gms.. .16

Digestchours at 46j' c. Y

' pH during mst digestion 1.1

Water Y* mi" soo,

v chloride gms.- .'75

Gelatin ems-- 1o 5 FORMULA VII Solution #1:

Potassium chloride gms 24.4 water mn- 474 m Potassium bromide gms 36.8 Ylilormal hydrochloric acid ml 71 Gelatin --gms 75 Temperature C 75 w Solution #2:

Siver nitrate gms 'Z5 Water ml '15 Temperature C 'I5 vwere arranged on a radius of Halide concentration after vprecipitation molan- .9

pH during second digestion. 4.4 Halide concentration during second digestion mnlar .09

The speed of this emulsion is approximately that oi Formula III, and vits gamma value is about Le.

The centrifuging hereinbeiore referred to may be carried out in, any suitable apparatus of welllmown type. 'For example, the emulsions of Formulae Nos. Viaud Vm, supra, were centrifuged in a bottle-type centrifuge, in which the bottles The machine was given a speed of about 20o() R. P. M. In. the case of Formula VI the cen-` trifuging was conducted for about ten minutes. Emulsions similar to Formula VH1 usually renuire about three to tive minutes.V

The soluble halide in the gelatin in which the centrifuged material is picked upis provided for the purpose of insuring the retention oi the reversing properties conferred by precipitation and ilrst 'digestion by preventing undue stripping of adsorbed halide from the silver halide 'grains during second digestion.

Halide need not be added to theplck-up gelatin referred to in connection with washed emulhas a speed about i .tion of halide, and by sions hereinbefore described since the washing is sufficiently curtailed to avoid removal of halide to such an extent as to result in undue stripping of adsorbed halide from the grains during second digestion.

The foregoing facts, hypotheses. and illustra# tive formulae have enabled us. andwill enable "others to better understand permissible variations in composition and manufacture of direct positive emulsions embodying our invention.

pH and concentration o! halide during grain t formation The concentration o! soluble halide in the solution before precipitation, after precipitation and during rst digestion, and the pH oi the solution throughout these stages of manufacture may be considered together in l view or their interrelationship as hereinafter more particularly noted.

The precipitation and first digestion are very similar processes. In precipitation the grains grow by the accumulation o1 the silver added to the solution. During first digestion at least some of the grains grow and itis likely thatall oi them undergo some solution and reprecipitation of silver halide on their surface, so that physically the two processes are very similar.

Thus we` may achieve substantially the same results by a larger concentration oi halide in the beginning of the precipitation, digestion with a considerably smaller concentraemploying an acidity of about pH 2.5 to ewe may reduce the halide concentration materially and still obtain satisfactory results.

while goed results may be obtained by adding the acid and halide excess after precipitation and .after washing or centriiuging they will produce substantiallyk no beneilcial eii'ect so far as reconcerned.

about siX inches.

as by prolonged situation. y

Despite the close association between the vari- Y bromine at thestart of precipitation be s The effect of the variables is also modied by the halide used; thus pure.l chloride emulsions can be successfully made by different formulae than pure bromide emulsions. The effect of the mixture of the two and of the percentage of iodide in bromide emulsions will further modify the ables, however, certain basic values emerge from our data.

In the case of pure chloride reversing emulsions (such as our Formulae I and V), we have found it important` that the pH of the emulsion in the grain forming stage be below the iso-electric point of the gelatin, i. e., below 4.7. The same has been found to be true of emulsions containing both chloride and bromide salts where the amount of bromide does not exceed about ten molecular percent. At -pH values below v4.7 we have made pure chloride` emulsions having a molar concentration of chlorine of .5 at the start `of precipitation and of.27 at the end. 'I'hese molar-concentrations include the chlorine present in the hydrochloric acid used to lowerthe pH, which in the aforesaid emulsion was approximately 2.7. Our experience indicates that for a purechloride emulsion to be effective for improving the density ratio so that a density ratiol of at least 4 can be attained, a molar concentration of chlorine of at least .2 is required after precipitation and during lfirst digestion.

As the percentage of bromide increases in the emulsion, the 'pH becomes less important and it -is possible to make an emulsion at pH valuesas high as 7. However, even withincreased percentages of bromide, a iow pH,-though not necessary, is found to result in improved density ratios.

The use of acid has a further desirable eii'ect in that it permits the use of higher concentrations' of halide during precipitation and first digestion without causing coagulation of the precipitated grains. The reason for this is not understood,

but the effect is substantial and permits a muchl wider choice of concentration.

The concentration of soluble halide in the solution before precipitation may vary considerably. Our data indicates an optimum value range of from about .7 to about 1.8 moles per liter of halide concentration before precipitation; At a concentration of about 2 moles per liter coagulation is frequently encountered. The data above referred to was derived from emulsions which were given first digestion. From our experience, we prefer that the concentration of greater than 1 molar in a bromide emulsion. l

With further reference to halide concentration during first digestion, we have found it desirable that inthe case of emulslonscontaining over fifteen molecular percent of bromide, the halide concentration should be in excess of .3 molar in order for the digestion to be effective in favorably modifying the density ratio. i

The eect of increasing halide concentration during first digestion is to increase the density in any 'case be determined by the point at which the grains coagulate. Coagulation is a readily recognizable Phenomenon. While affected by the halide concentration, it may be noted further that coagulation is also modified by the gelatin concentration, the greater the concentration of gelatin the less the tendency of the silver halide l'ainstocoagulate. K

As an example of the effect of pH value and gelatin concentration on coagulation, we have found that at pH values above the iso-electric point (4.7) of gelatin, coagulation will take place during first digestion with a halide concentration of .3 molar or above and with a gelatin concentration of 16 percent, whereas, under the same conditions, with a pH of 3.2 the concentration may be carried to .6 molar without serious co` agulation.

'Coagulation also depends upon vsolution of the grain and involves a time element. Forinstance, in bromo-iodide emulsions, precipitated and digested at a rather high temperature (95? (1.), the first digestion being only ten to twelve minutes, coagulation will not take place during first digestion even at halide concentration of one molar and above. Y

We mayfurther point out thatthe silver halides are partially soluble in solutions of soluble halides and that the concentration of soluble halide should behigh enough for the silver halide to be definitely soluble therein (as determined by a definite change in size or shape of the grain as digestion proceeds) for tirst digestion to be effective for improving the density ratio.

In general, the silver chlorides should be precipitated and digested at alow pH (below the iso- The presence of a larger percentage of iodide (one molecular per cent or above) causes an increasing loss of silver em'ciency. The latter may be defined as the ratio of the maximum developable density to the silver coated per unit area of `the coated surface (in terms of grams of silver per square meter) or, in other words, as the percentage of the total number of grains per unit ratio. The upper limit of the concentration will 'area which will develop out under the standard conditions of development after exposure oi the coated emulsion to that point on the sensltometric curve which corresponds to the maximum density. The loss in silver eillciency due to increase in percentage of iodide may be partly offset -by lowering the halide concentration, but the effect becomes so powerful at 5 molecular percent of iodide that -it is found practically impossible to make a satisfactory reversing emulsion.

Concentration of gelatin during train formation ployed, whereas` in bromide and bromo-iodide emulsions the gelatin concentration may be as low as 1%% or 2%. Indeed, for bromide and bromo-iodide emulsions it is found distinctly desirable for optimum reversing properties to utilize low concentrations of gelatin during precipitation and first digestion; and the evidence indicates that the gelatin concentration may bead- 'vantaseously cut down to the point where it .lust

suspends the precipitating silver bromide. This accords with the fact that gelatin is preferentially adsorbed tothe silver bromide and tends to inhibit the above mentioned adsorption of other ions, such as bromine, and hydrogen. In fact sufd'ciently cutting rdown gelatin and increasing bromine or soluble bromide, makes it possible to dispense with hydrogen or equivalent ions, as will be explained later.

Time and temperatura. of first digestion It may, indeed, be observed further that slow' precipitation may be substituted in whole or in part for ilrst digestion. The slow precipitation permits time for the adsorption of halide in the crevices and irregularities in the grain as it is being enlarged in size by the further addition of silver.Y Excessive rst digestion raises negative fog and diminishes positive contrast.

The temperature during precipitation and first digestion is variablev through a wide range as indicated b y the foregoing examples of our emulsions. Prolonged digestion atv low temperature has substantially the same effect as shorter digestion at higher temperature.

Character of the metal ofthe soluble halide salt We havefound that ammonium, potassium,l

and lithium are equally suitable as the cation or metal of the soluble halide salt. They are `preferable to sodium, since the latter gives a higher minimum reversal density.

i Nickel, zinc, or strontium may also be utilized. as the metal of the soluble halide salt. at least in amounts up to about 25 molecular percent. Copper may also be employed.

C'haracter of the acid or equivalent substance The hydrogen ion is the effective constituent of the acid used and the character of the anion has small eifect. We have successfully used hydrochloiic, hydrdbromic, nitric, sulphuric, phosphoric, and -citric acids. Acetic acid was not satisfactory because with the large amounts required to obtain the desired low pH, it decom- `tion.

posed the gelatin.

The different acids modify somewhat the shape of the photographic curve and to some extent the speed of the emulsion. In the case of bromide emulsions, for instance, comparing hydrobromic acid with hydrochloric, the former will produce an emulsion about one and one half times faster, but with a lower contrast and slightly lower minimum reversal density. As noted hereinbefore` (as exempliiled by formula IV-)3 positive ions other than hydrogen may be' used, though we have not found any as eiective or convenient as hydrogen. i

It will, of course.-be apparent from what has been noted hereinbefore that ifthe acid contains halide ions that are relied upon to increase the halide concentration to a desired value, a change tosulphuric 'or other acid not containing lalide should be compensated by increasing correspondingly the amount of solublehalide salt.

With further reference to einem-positive' ,10115 other than hydrogen, rfor promoting the adsorption of halides to eiect reversal, we iind that such ions should preferably Ibe close toy hydrogen in the electro-motive force series. The strongly positive alkali metals such as potassium and sodium are not desirable for this purpose, whereas nickelv and silver are suitable. Furthermore, the

metal or electro-positive ion, must be' chemically compatible with the solutions used in precipitation or iirst digestion. Lead and mercury, for example, produce insoluble halide salts and would be thrown out of solution. In addition, the metal must not be photographically antagonistic." The use of iron, for instance, is not indicated because of its deleterious effect on the normal characteristics of an emulsion.

The valence is' not important, as is evident from the fact that the nickel in the salt used in Formula IV was bivalent, whereas hydrogen and silver are monovalent. However, it will be apparent that a metal should not be used the valence of which might produce an oxidizing reaction, since this would diminish the normal photographic sensitivity.

Concentration of silver nitrate and time and manner of its introduction 'I'he prime effect of the concentration of silver nitrate is to change the ratio of halide con-l trast negative emulsions.

Stirring The eiect of rapid andy effective stirring which is preferred, is to produce a more. uniform emulsion, and, in general', emulsions of higher contrast. We have found that effective stirring during precipitation produces a more uniform grain size, a higher contrast, and a better ratio of maximum to minimum density in the nished emulsion. The improvement in density ratio due to stirring may be explained on the basis that thoroughv stirring brings the silver halide grains more effectively into intimate contact with the excess halide of the solu- Removal of soluble salt Washing or centrifuging should accord with our theory that the silver halide grains should not be to'much stripped of their associated reversing material. We have found that if the soluble ,halide concentration is reduced below .01 molar the reversal characteristics become very poor and at .005 molar they are practically destroyed. In other words, it is our'experience that if the soluble halide concentration is reduced by removal of soluble salts to avalue lying somewhere between .005 and .01 molar, a practical reversing emulsion will no longer result. For purelchloride emulsions, or broma-chloride emulsions containing large percentages of chloride, it is preferable that the soluble halide concentration should not be reduced below .02

'molan With emulsions containing a substanltial amount of bromide (as for example a pure bromide emulsion or a broma-chloride vemulsion of about 70v molecular percent bromide) the soluble salt reduction should preferably not be carried below a point around .01 molar as indie cated above.

Our usual practiceis to shred -the emulsion into "noodles" approximately V4 inch square and wash with distilled water. ilve baths of ilve minutes each. The "noodles" are not stirred while'the water lies on them but are shaken up when the water is poured oil' at the end of each bath.

`Our practice in centrifuging is to centrifuge onlyonce. That is, the grains are thrown down,

I the solution used in iirst digestion poured off,

and the grains are picked up in a fresh gelatiny ysolution containing some soluble halide.

- extent commonly used for negative emulsions has resulted in emulsions that were badly logged and which showed no longed exposure.

v Second digestion The eiIect oiv second digestion is most clearly marked in our pure bromide and bromo-#iodide emulsions. These emulsions, after washing or centrituging. almost invariably show a very fiat curve with negligible, maximum density. Second digestion causes a marked increase in density ratio by raising the maximum developa-ble density, which increase is eilected without any large' increase in mininmum density. Excessive second digestion iinally raises the' minimum density and causes a substantial loss in speed.

Second digestion does not cause a reduction of minimum obtainable reversal density. Consequently, the potentialities oi' the emulsion may.

, be determined, alter removal o! soluble salts and` before second digestion, upon the basis of what may be termed "silver ratio," deiined as `follows:

silver ratio=the ratio of grams o! silver coated per square meter to minimum reversal density Ii' precipitation, nrst digestion, and reduction of amount of solublesalts have beenproperly performed, the silver ratio at this 'point will be at least 4 to 1; and ii said ratio is less than this value, second digestion will not produce a practical reversing or positive emulsion..

We have not found that the gelatin concentration in second fdigestion has any eilect on the photographic characteristics other thanI those f which could be accounted lor by 'diilorences'in the rate o! development due to thicker ner gelatin nlm in-the coated product.

or thin- Jlhe lower limit o! soluble halide during secfond digestion is thesame as that given above under fRemoval ci' soluble salts. The upper limit is the concentration at which the silver halide grains become appreciably soluble. t

reversal, even after prol this upper limit the enect becomes the saine as additional nrst digestion. The rapidity with which the emulsion digests to the optimum point varies with the halide concentration, being more 5 rapid, the lower the concentration.

'I'he time and temperature of second digestion are not critical. In pure chloride emulsions itis frequently unnecessary to employ second digestion at all. the vdensity ratio and contrast being suillciently high after reduction of the amount of soluble salts. Our usual practice in bromochlorides containing from 25% to '10% (molecular) of bromide is to digest for two to four hours -at 46 C. Pure bromide and brcxno-iodide emulsions usually require somewhat longer digestion to develop thedesired density ratio at the above `noted temperature.

As previously indicated. in connection with Formula III, the inclusion of'thiosinamine in second digestion is not effective for increasing the sensitivity of our positive or reversing bromide emulsions, unless the thiosinamine is added in quantities much greater than Vin negative emulsions. It is believed that thiosinamine retion, togetherwith As a result, the presence of high concenl acts with silver bromide in a complex manner to form silver sulilde as an end product of reachydrogen ions andl bromine ions. trationsof bromine ions, such as are present in our bromide and broma-iodide emulsions, and-the relatively high concentration of hydrogen ions which we frequently employ, deters the reaction between the thiosinamine and silver halide. In our bromide and bromo-iodide emulsions we pre fer to employ as much as 1 part ot thiosinamine to 1500 parts of gelatin.

With respect to pH values during second digestion, the pH should-not be above 4.7 for pure chloride emulsions. Using only a very small percentage of bromide. however, makes it possible to raise the pH to 7 or above during second digestion. l Y

The desirable value oi' pH during second digestion is determined by several factors. The addition of acid lowers the viscosity of the emulsion and this should be taken into account in adjusting the coating conditions. In practice, in sec-v cyanate withthiosinamine (as in Formula VIII,

Solution #3) enables excellent sensitizing results to be attained at lower pH values than are pos# sible when thiosinamine alone is used.

' Other sensitizers. less ailected by hydrogen and bromine ions than is thiosinamine, may also be useful. Some known sensitizers, such as thiourea, have a tendency to produce fog. but this is not necessarily a prohibitive disadvantage in direct positive emulsions.

Further characteristics of our emulsions Our emulsions havethe following additional characteristics whereby we lbelieve them to be uniquely identified. l

l. When our emulsions are washed in pure waterfor a few minutes before exposure, and `then dried, exposed and processed. it will be found that` their reversing propertiesare praetioally destroyed'.

then dried, exposd and processed, it will be found y i `that the reversing properties have not been substantially modified.

' coating Coating of our emulsions may be conveniently carried Iout with conventional coating machines. The customary coating agents may be employed, suchl as formaldehyde and chrome alum for hardening the gelatin and'rendering it insoluble; soap bark or saponin to cause an even spreading and wetting on the support by the emulsion and a1- cohol to inhibit frothing.

Coating on glass, Celluloid or other supports impervious to water presents no problem not met with in negative emulsions. However, when our emulsions are coated on paper a new problem arose in that the density ratio was lowered due to 4a marked rise in the minimum obtainable density. This was found to have resulted from the absorption by the paper of halide ions from the emulsion and may be remedied by passing the paper tobe coated through a strong solution of soluble halide, and thereafter coating with the emulsion. Before coating the paper may be dried or it may be coated while still wet.

Alternatively. the amount of halide which the paper will absorb may be determined and the calculated quantity of halide added to the emulsion shortly before coating. We do not claim the aforesaid improvements in coating on paper or other molsture-pervious support as our joint invention, since said improvements are thel sole invention of George B. Crouse. e

Tests for determining potential reversing mperties after vremoval of soluble salt, for any errors in formula or procedure occurring in prior operations, in the preparation of our emulsions, and it is therefore highly desirable to have a means of testing the potentiality of the emulsion during early stages of manufacture. The simple procedure outlined below furnishes the means of accomplishing this object.

The test may be applied at any time afte'r precipitation and before washing or centrifuging to determine whether thel formula and manipulation are correct up to that point. The test is based on the fact that a formula capable of making. a practical emulsion embodying our invention, will, before the soluble salts are removed, show distinct solarization when exposed in the wet state. The test is performed as follows:

Approximately .5 m1. of emulsion is withdrawn from the kettle and coated on a 1" by 3" microscope slide, or equivalent area. The slide is chilled and immersed in pure cold water for 5 to 10 min` utes (5 minutes for a chloride emulsion and l0 minutes for a bromide. emulsion). It is then immediately exposed in the wet state either` through a. step wedge or by means of exposures varied in time. The exposures are carried to suiiiclently large values to determine whether or not the emulsion definitely shows solarization Whenvdeveloped under convenient standard conditions.

If the emulsion cannot be made to show definite solarization at any exposure, the formula and manipulation are incorrect. If definite solarization is visible, proper after treatment will result in a practical reversing or positive emulsion.

To give an illustration of the manner in which this test may be utilized, the following may be noted: v

Suppose that it is desired to produce a reversing emulsion of a given type, and that we start with a conventional negative formula. The test` may be applied-to the emulsion after precipitation and again after a reasonable amount of first digestion. The test will, ofcourse, show negative results, that is, no solarization will be visible.

One o f the conditions for producing a reversing emulsion embodying our invention isto provide. suicient halide concentration during precipitation and/ or first digestion. Successive trials may be made, increasing the halide concentration and applying the test each time until the test shows a positive result, which will fix, for a particular formula, the minimum necessary halide concentration. l

Then, after first digestion and removal of soluble salts,we can apply a second test to determine whether the minimum limit for the removal of soluble salt has been overstepped or not. For this purpose, a plate or microscope slide is coated with a quantity of said emulsion (following removal of soluble salts) to provide a known amount of silver per unit area, chilled, and dried in the usual way.

Sufficient exposure is then givento the plate to produce upon development the minimum obtainable reversal density. This plate.- when dee veloped under standard conditions, as stated below, or equivalent, should show a ratio of grams of silver per square meter to minimum obtainable densityvof 4 or more. If it does so the emulsion may then be given second digestion to produce the best density ratio. If the ratio is less than 4 no subsequent 'treatment will produce a practical emulsion.

The above solution is diluted with water, 1:1. `Plates are developedgat 65 F. for 3 minutes, the tray being rocked by hand.

Theory As previously noted, our theory as to the rea sons why reversal is accomplished in our emulsions is that halide ions become intimately associated with or adsorbed to the silver halide grain, and that when light of suillcient intensity acts upon such a grain the halide is liberated in greater proportion than metallic silver. The concentration of halide ions around the latent image thus builds up to the point where it is suflicient to combine withthe liberated silver and thusY destroy the latent image. We will not here repeat the evidence, hereinbefore referred to, which appears to support our theory, but we will note briefly certain further considerations which lend it further weight. Of course, whether or not our views are correct as to the manner in which the latent image is destroyed is lnot material, but an explanation of the possible production of the reversal edect in our novel emulsions on the basis of the evidence at hand may be helpful in an appreciation of certain aspects of our invention. i Obviously in those of our emulsions which are made at relatively high pH values (above the isoelectric point) no new material is added to cause reversal, and all of the evidence indicates that it is the excess halide which brings about the reversal characteristics.

Further, it is not the mere presence of the excess halide in the emulsion which is effective since' theaddition of halide to a non-reversing emulsion will not cause it to reverse unless the grains are actually digested insuch high halide concentration.

The matter may be viewed from the concept of 4a force or potential existing between the silver halide grain and the associated halide ions or halide containing molecules. To make this clearer. consider the behavior of a bromide emulsion throughout its manufacture.

With a. properly balanced formula, and given the correct amount of first digestion, a -test plate made and tested as directed under Tests for determining potential reversing properties, above, will show pronounced solarization in. the wet state. When this emulsion is washed or centrifuged and coated, it will be found that it will produce only a very low ratio of maximum to minimum density, and prolonged digestion in a solution less rich in halide than that used during rst digestion is'necessary in order to produce a satisfactory density ratio through the increase in the maximum developable density.

, On the other hand, if instead of the usual washing procedure, prolonged washing is employed, it

' will be found that the emulsion, before lsecond digestion, has substantially the same characteristics as before. However, very short second digestion is sumcient to separate the adsorbed halide from the grains and it is practically impossible to produce a sa-tisfactory emulsion because the sec! ond digestion time becomes extremely critical.

Referring again to this same test, our finished emulsions will stand very little washing before exposure, and also will show substantially no reversal when 'exposed\wet, whereas, at any point before the removal of soluble salts, the test previously outlined herein will show solarization. This may be accounted for on the basis that an emulsion in which the adsorption of reversing agent after precipitation or first digestion is sufficiently great to reverse in the wet state, will retain the adsorbed ma-terial on its silver halide grains with sufiicient tenacity so that reversing properties will not be lost when the concentration of soluble halide or other reversing agent in the solution surgrains reverse too quickly` before others have had a chance to build up density and the result is a low overall densit If the bond-is too weak, the emulsion becomes unsatisfactory because a. large percentage of the grains then fail to reverse at all, producing a high minimum obtainable density.

It is probably theoretically possible to soadjust conditions in precipitation and first digestion that this bond would have the correct value to produce a. practical emulsion without second digestion,

, and, as a matter of fact. wehave accomplished must be given to satisfactorily increase the density rounding the grains is reduced by washing or cention of halide in the environment weakens the bond which is still further weakened by digestion or heat treatment.. 'Y

There is thus a best value of the strength of this bond; if the bond is too strong some of the high speed.

this in pure chloride emulsions and in emulsions containing small percentages of bromide. In the pure bromide emulsions we have not been able to do this, and second digestion is required to adjust the factor to a value which producesa practical finished product. l

Further strength is given the theory -by the dif-- ference in behavior between chloride and bromide l emulsions. Itis well known that the strength of adsorption between silver chloride and the chloride ion is i'nhercntlyweaker than between silver bromide andthe bromide ion. Thus the mere removal of the bulk of soluble halide from the chloride emulsion is generally enough to weaken the l bond so that a satisfactory silver eiiiciency is obtained. In bromide emulsions where the bond is strong it is generally advisable to long second digestion. i

We may also point out that in line with the idea just set forth, the reversing properties of chloride emulsions are more easily destroyed than those of bromide emulsions. Thus, as we have previously indicated, the halide ion concentration should not be allowed to reach as low a value after removing the soluble salt and during second digestion, in a chloride emulsion, as is permissible in an emulsion containing a. considerable quantity of bromide.

Summing up conditions favorable to good reversal qualities, it appears: First, precipitation and first digestion must be under such conditions that in addition to the amount of halide necesremoves the soluble-salt products of precipitation.l

Third, the-part ofthe halogen in the gelatin which remains in the immediate neighborhood of the grains, must be diffused through the bulk of the gelatin during second digestion, without stripping-the grain itself of halogen. Fourth, in the case of the bromide and broma-iodide emulsions, sumcient sensitizer and second digestion ratio and also normal speed of the emulsion so that on each exposed grain the latent image ,nucleus will'be built up to the point of reversal athigh speed. Still more generally stated, we

must` (a) adsorb halogen on the silver halide grain; (b) get rid of the excess soluble-halide in the bulk of the gelatin without losing the adsorbed halogen from the grain, and (c) in the case of brcmideand broma-iodide emulsions. we must use sensitizers ln unusual amounts to obtain Fonutm IX As further illustrative of variations in formulae we may notethe fol1owing.'in which conditions favorable to the adsorption of halide are preserved and good results are attained, without the aid of positive ions (such as hydrogen ofthe employ relatively acid in Formula III or nickel of the nickel chloride l in Formula IV) For purposes of t illustration. and comparison, the above formula is the same and the manufacture is the same as for Formula III, except that As expected,- the emulsion resultingk froml Formula V is substantially the same in photo- -f graphic characteristics as that produced by Formula III, except that reversal speed is somewhat slower, and minimum density somewhat higher.

In the FormulaIX emulsion the pH was 8.1.

The halide concentration before precipitation was .881amolar and during iirst digestion was .35 mo r.

Substances other than'halides It will be apparent `that substances other than halides may act in like manner if suitably associated with, or adsorbed to, the silver halide grain, and, as a matter of fact. we have found that the sulfocyanate ion may be used in this manner. The sulphocyanate ion is capable of combining with silver te eiect reversal as illustrated by way of example in the iollowins formula, wherein potassium suli'ocyanate is substituted in part for the excess halide in the solution during precipitation and first digestion:

Heat Solution #1 to 16 wand add Solution #2 in three andthree-quarters minutes.

First digestion. 5 hours, l5 C. l g

Centrifuga and pick-up in: i

Weit mi 676 Potassium chloride (10% soiution) ml 15 Gelatin gms 2 5 Store overnight. conveniently in a refrigerator Ammonium bromide ..-gmsn 12.48 at about 40F- Potassium i0d1de mgm.- .os Dise 3/2 hOmS at 46 C- Solution B: Halide concentration before precipitation Silver nitrate-1 sms-- 10 10 y molar 1.34

Watertomake m1 110 After precipitation do .es

Solution C:

vAmmonium bromide gms.. 5.76 ll'gimmg Second digestion 6 Water tomake ml" no de concentration during second diges- Bromme gm" .005 15 CNS concentration beforeprecipitation Solution E: The reversal speed of this emulsion is approxi- Thiosinamine -gm-.. .017 mately that of emulsion VII, hereinbefore re- Water to make (about) ml..- 2 20 ierred to andits contrast is intermediate that oi emulsions VII and VI.

We have found that potassium sulfocyanate, when used during iirst digestion for promo-chloride emulsions (of the order of about molecu lar percent bromide or over) raises contrast and maximum density. However it has not been found suitable for use during first digestion in the case Aof brume-chlorides of substantially lower bromide content (25 molecular per cent bromide, for example).

Among the many advantages of our new direct positive emulsions is that theymay be exposed and processed with the same ease and simplicity as an ordinary negative emulsion.

(l) Exposurev may be carried out ln a camera, projection printer or by contact with a transparency. Positive copiesmay be made directly from vpositive originals.

(2) The exposure is adjusted to fall within the range of the' material asiin frdinary emulsions. The effect of an error in exposure is different with the direct positive material, however, than with negative materials.. In the case of the latter. over-exposure will produce animage which is too dark all over. In the case of the direct positive emulsion, over-exposure will produce an image which is too light, while under-exposure will produce an image too dark, which may, for gross under-exposure, be a negative.

(3) The direct positive emulsion may be developed in any of the standard developing solutions, using standard technique. Variations in the composition of the developing solution will have much the same effect as with negative emal.

sions.

Ally-over exposure- Uniform all-over exposure merely adds a deilnite uniform amount of light to all the diiierent amounts of light received from diiierent parts of the object during image exposure: and theflect is the same whether the uniform light is added` before. during or after imase exposure. Consequently. the amount of light contributed `by the additional uniform exposure, mutA be rezulated with respect to the licht contributed by the image exposure. so that the sum total will have tbe proper value with respect tothe reversal curve between maximum Fonxmx Solution #1: .Y Gelatin -gms-- 100 Normal hydrochloric acid --..-.'.-ml 87.5 Potassium chloride ..-..-ms..- 27.7 Potassium bromide sms 31.6 Potassium suliocyanate (solution-82 l me. per ml.) --.ml... 80 Water m1 420 Solution #2: v

Silver nitrate..- ..ms.- 'l5 Water ml.- 'l5 density pointfl and minimum density point 8. Fis. 3.

In the case of emulsions in which reversal takes place only after a long maximum density reuse. Yas indicated by the line between I and 2z, Fis. 1, it has been proposed to sive'the emulsion a preliminary all-over darkenine to the point 2z, either by al1-over exposure ory by chemical i 2,401,061 order to reduce the time required for the image.

We may use such regulated over-all exposures to produce other effects, as follows:

(1) We may use it to decrease contrast in the developed positive, as is indicated by the flattening of the slope of the image plus overall" curve 2b-3, Fig. 3, as compared with the steeper curve 2-3 for image exposure only. l

(2) As shown by the dotted line, df-, Fig. 3, a greater amount of over-all exposure will decrease maximum density, in addition to further decreasing contrast.

(3) The range or latitude of image exposure which can be faithfully reproduced in the devel-V oped positive, is increased, as indicatedby the shift ofthe Z-Za line of reversal density. for

i image exposure only, backward along the log exposure line, to line 2li-2c which is maximum density for fimage plus overall.`

(4) `As a corollary of the foregoing, an image exposure of such latitude as to produce only a partial negative image in the shadows, may be converted to a good positive, by a proper amount of over-all exposure.

The following are examples illustrating practical advantages oi' our emulsions, for certain special purposes:

(1) In rotogravure printing, the original copy is usually received in the form of photographs, drawings, etc. (a) These are re-photographed to form a negative.' (b) A contact' print of this negative is made to form a positive transparency. (c) This transparency is' then laid down onthe sensitized carbon tissue, previously printed through a' rotogravure screen. (d) The carbon tissue is exposed through the transparency, then (e) developed in warm water. (f) Through the resist thus formed, the ilnal printing cylinder is etched.

(a) For such purposes. use of our emulsions obviates making e. negative, the positive transparency being produced, ready for laying down on the carbon tissue, by photographing it directly 1 5 contrast can be varied is by varying the development. and it will be obvious that where scene and sound on the same nlm require dinerent contraste, it is very dimcult to give them the required different developments.

With our direct positive emulsions, it is a simple matter to give one of the areas an over-all exposure of the amount required for the desired contrast in that area, without affecting the other area.

(3) The latter method is also very useful in printing still pictures. For instance, in printing -from the original picture. This saves labor, de-

generation of the image, and time between the receipt of .the copy and completing of the cylinder.

(b) In the positive transparency, good and had Y originals must be brought to substantially the by projection, local reduction o! density may be secured by the local .application of light from a small dash-light. That is, the picture may be more or less locally re-touched by use oi a tiny dash-light, with or without the aid of masklng to localize the ellect.

As concerns the meaning of some of the terms used in our claims, it will be recognized that, in producing the latent image the light acts to cause separation between some of the silver and the halogen with which it was combined, and, as concerns the silver, this is a reducing action. The eiect of the further light, whereby the latent negative is destroyed, is a chemical reaction which is the reverse of the primary reducing action, and so, in the chemical sense, it is properly describable as an oxidizing reaction, although it scribed or portions thereof, but recognize that various modloations are possible within the scope of the invention claimed.

We claim:

1. The process or producing e direct positive emulsion, comprising precipitating silver halide grains in a solution containing gelatin; including in said solution a soluble halide additional to said silver halide grains and capable, upon suilicient exposure to light, of destroying the developability of said grains; digesting said solution to permit said grains to'grow in said solution in the pres-- ence of said soluble halide introducing into said emulsion prior ,to completion oi' saidV digestion r suilicient acid to produce a pH lower than 4.7:

(2) In talking motion pictures, it is common i to print the sound record alongside the pictures onthe positive used for projection. In one system the sound variations appear as varying A densities in a strip of un'iiorm width, and in `another system they appear as varying areas of uniform density; but'whichever system is used,

many cases` will arise in which the density range and ,contrast required for the picture will be widely different from that required for `the sound record. Where the exhibition positive is printed from a negative by ordinary negative-producing methods as at present, the only way in 'which said soluble 'halide being present in such concentration during said ,digestion that upon exposure of the emulsion in a moiststate and subsequent development, after said digestion; and prior to reduction `of soluble salt concentration, solarization is exhibited; and reducing the soluble salt concentration while maintaining said soluble halidey in the emulsion in a concentration of at least .01 normal despite said reduction.

2, The process oi producing a direct positive emulsion,- comprising precipitating silver halide grains in a solution containing gelatin; including in said solution a soluble halide additional to said silver halide Vgrains and capable, upon suilicient exposure to light, of destroying the developabillty 'ot said digesting said solution to permit said grain', to lgrow in said solution in the presence of said soluble halide: the VpH: oi' aidiolution being not substantiallyhigher than 7 during said precipitation and digestion; said soluble halide being present during said digestion in a concentration less than suihcient to cause coagulation of said grains but sumciently high that upon exposure of the emulsion in a moist state and subsequent development, after said digestion, and prior to reduction of soluble salt concentration, solarization is exhibited; and reducing the soluble salt concentration while maintaining said soluble halide in the emulsion in a. concentration of at least .01 normal despite said reduction.

3. The process of producing a direct positive emulsion, comprising precipitating silver halide grains in a solution vcontaining gelatin; including in said solution a substance capable, upon sumcient exposure to light, of combining with metallic silver and destroying the developability of said grains; digesting said solution to permit said grains to grow in said solution in the presence of said substance; lthe pH of said solution being not substantially higher than 7 during said precipitation and digestion; said substance being present during said digestion in a concentration less than suiilcient to cause coagulation but suillciently high that upon exposure of the emulsion in a moist state and subsequent development, after said digestion and prior to reduction of soluble salt concentration, solarization is exhibited; and maintaining said substance in such concentration, despite reduction ot soluble salts, as to produce a ratio of maximum to minimum reversal densities of at least 4 to 1 in the finished emul- V sion.

said grains .to grow in said solution in the presence of said soluble halide; the PH of said solution being not substantially higher than 7 during said precipitation and digestion; said soluble halide being present during said digestion in a concentration less than suiilcient to cause coagulation and not less than .2 normal; and reducing .thesoluble salt concentration while maintaining said soluble halide in a concentration of at least .01 normal despite said reduction.

5.l The process of producing al direct positive emulsion, comprising precipitating silver halide grains in a solution containing gelatin; including in said solution, a soluble halide additional to said silver halide grains and capable, upon suilicient exposure to light, oi' destroying the developability of said grains; digesting said solution to permit said grains to grow in said solution in the presence of said soluble halide introducing into said emulsion prior to completion of said digestion a substance ailording electro-positive-ions ior promoting intimate association ofy said soluble halide with said silver halide grains; the pH 'of said solu tion being not substantially higher than 'I during said precipitation end-digestion; sind soluble haiide being present during said digestion in a concentration lese than sutlicient to` cause coagulation but sulliciently high that upon exposure of the emulsion in a moist state and subsequent development, ai'ter said digestion. and prior to reduction of the soluble salt concentration, solarization is exhibited; and reducing the soluble-salt concentration while maintaining said soluble halide in the emulsion in a concentration of at least .0l normal despite said'reciuction.

6. The process of producing a direct positive emulsion, comprising precipitating silver halide grains in a solution containing gelatin; including in said solution a soluble halide additional to said silver halide grains and capable, upon sulcient exposure to light, of destroying the developability of said grains; digesting said solution to permit said rains'to grow in said solution in the presence of said soluble halide; the pH of said solution. being not substantially higher than 'I during said precipitation and digestion; said soluble halidebeing present during said digestion in a concentration less than sumcient to'cause coagulation of said grains but sufilcientiy high that upon exposure of the emulsion in a moist state and subsequent development, after said digestion, and prior to reduction of soluble salt concentration, solarization is exhibited; and reducing the soluble salt concentration while maintaining said soluble halide in the emulsion in a concentration of at least .0l normal despite said reduction; and thereafter digesting said .emulsion to raise the maximum density to a desired value.

7. A photographic emulsion layer adapted upon exposure and development to produce a direct positive image, said emulsion including silver bromide grains capable of being rendered developable under the action of light, and a soluble halide capable, upon suicient exposure to light, of destroying the developability of said grains, said soluble halide being present in the stage of grain formation in a concentration not less than .3 normal, and beingretained in the finished emulsion, after reduction of soluble salt concentration, in a concentration not less than .01

normal.

8. A photographic emulsion layer adapted upon exposure and development to produce a direct positive image, said emulsion including silver chloride grains capable of being rendered developable under the action of light, and a soluble halide capable. upon sufficient exposure to light, o! destroying the developability of said grains, said soluble halide being present in the stage of grain formation in a concentration not less than .2 normal, and being retained in the nn- ,ished' emulsion after reduction of soluble salt soluble halide capable, upon sumcient exposureto light, of destroying the developability of said grains, said soluble halide being present in the stage of grain formation in a concentration not less than .2 normal and being maintained, notwithstanding reduction of soluble salt concentration, in a concentration not less than .0l normal in thel finished emulsion.

l0. A photographic emulsion layer adapted upon -exposure' and, development to produce a direct positive image, lsaid emulsion including silver bromide grains capable of being rendered developable under the action of light, and a soluble halide capable, upon suiiicient exposure to light, of destroying the developability of said grains, said soluble halide being lpresent in the stage of grain formation in a concentration less than sutllcient to cause coagulation of the grams,-

and not less than .'3 normal, and being retained in the finished emulsion. after reduction of soluv ble salt concentration. in a concentration not grains, said soluble halide being present in the stage of grain formation in a concentration less than suicient to cause coagulation of the grains and not less than .2 normal, and being retained in the nished emulsion after reduction of solu- -ble salt concentration. in a concentration not less than .01 normal, said emulsion being acidined during the stage of grain formation to a pH below 4.7.

GEORGE B. CROUSE. FRANCIS A. HOLT. JACOB L. JATLOW. 

